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Extension > Garden > Commercial fruit and vegetable production > Plant diseases > Late blight of tomato

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Late blight of tomato

Anna Johnson, Michelle Grabowski and Angela Orshinsky


Late blight is a potentially devastating disease of tomato and potato, infecting leaves, stems, and fruits of tomato plants. The disease spreads quickly in fields and can result in total crop failure if untreated. Late blight of potato was responsible for the Irish potato famine of the late 1840s.

late blight infects

M. Grabowski, UMN Extension

Late blight infects leaves, stems and fruit

Host and pathogen

Late blight is caused by the oomycete Phytophthora infestans. Oomycetes are fungus-like organisms also called water molds, but they are not true fungi. There are many different strains of P. infestans. These are called clonal lineages and designated by a number code (i.e. US-23). Many clonal lineages affect both tomato and potato, but some lineages are specific to one host or the other. The host range is typically limited to potato and tomato, but hairy nightshade (Solanum physalifolium) is a closely related weed that can readily become infected and may contribute to disease spread. Under ideal conditions, such as may be encountered in a greenhouse, petunia also may become infected.


Signs and symptoms


Biology and disease cycle

large brown blotches with green gray edges

M. Grabowski, UMN Extension

Leaf infections are large brown blotches with a green gray edge

Phytophthora infestans can overwinter in Minnesota if protected in potato cull piles. Overwintering in a tomato production system is unlikely but infected tomato fruits may give rise to infected volunteer seedlings the following season. The most common routes of introduction each season are infected potato seed tubers, infected tomato transplants shipped in from other regions, or windblown sporangia (asexual spores) from the south.

Under cool, wet conditions, P. infestans can infect and produce thousands of sporangia per lesion in less than five days. These sporangia easily become air-borne, resulting in prolific spread of the pathogen. The disease can potentially destroy entire fields in a short period of time if left unmanaged. Long-distance spread to other fields is also likely, particularly under cloudy conditions.


Resistant varieties

Even a resistant variety will show some late blight symptoms when conditions are highly favorable for disease. Levels of resistance vary between cultivars and may be more or less effective depending on the P. infestans clonal lineage present. Several resistant varieties are listed below. Check seed catalogues and the Cornell vegMD webpage for new varieties with resistance to late blight.

Tomato varieties with resistance to late blight
Mountain Magic (F1)
Plum Regal (F1)
Defiant PhR (F1)
Mountain Merit (F1)
Iron Lady (F1)
Jasper (F1)
Red Pearl (F1)
Matt's Wild Cherry
Wapsipinicon Peach
Lemon Drop
Pruden's Purple

Cultural control

high humidity infection

M. Grabowski, UMN Extension

In high humidity, powdery white spores form on infected fruit, leaves and stems

Chemical control

Fungicides are available for management of late blight on tomato. Late blight does not occur every year in Minnesota. Growers should watch for late blight symptoms in their regular scouting, particularly with weather conditions that favor disease. Reports of regional outbreaks of late blight can be found at the USAblight website.

dry brown rot

M. Grabowski, UMN Extension

Infected fruit have a dry brown rot

Fungicide applications should be made prior to infection when environmental conditions favor disease to be the most effective. Phytophthora infestans is a water mold and not a true fungus. Fungicides specific to water molds must be used and applications repeated according to label instructions. For a current list of fungicides for late blight management visit the Midwest Vegetable Production Guide. Rotate fungicide groups and/or tank mix fungicides to avoid producing fungicide-resistant isolates.

Because late blight disease development is so dependent on weather, disease forecasting computer programs such as TOM-CAST have been developed to estimate when the pathogen is most active. The program uses temperature, humidity, and rainfall data from a weather station. The program will determine whether a fungicide application is necessary. Following this system rather than just applying fungicide every 7 days may save several fungicide sprays per season while still providing good disease control.


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